France's foreign ministry said on Saturday it was very concerned by the seizure of a British oil tanker in the Strait of Hormuz, saying such an action harmed de-escalation efforts in the region.

Iran said on Friday it had seized Britain's Stena Impero tanker, which had been heading to a port in Saudi Arabia and suddenly changed course after passing through the strait at the mouth of the Gulf.

"We have learned with great concern of the seizure of a British vessel by Iranian forces," the ministry said in a statement.

"We strongly condemn it and express our full solidarity with the United Kingdom."